Botrytis or gray mold is a fungal disease that often becomes prevalent after severe late spring freezes or during periods of cool, rainy weather. It can be especially damaging to plants when wet, humid conditions continue over several days.
Botrytis appears as a grayish or brown powdery mold covering flower buds, leaves, and stems. It affects many different perennials but is especially common on peonies.
As with leaf spot, botrytis overwinters in dead leaves and other plant tissue so it is important to remove all plant debris from the garden in the fall. If you notice botrytis on buds, leaves, or stems, carefully remove the infected plant tissue, place it in a bag and discard it in your trash. Never do this while the plant is wet or you risk spreading the disease to other healthy plants.
In the spring, just as the shoots begin to come up, drench the soil with a mixture of Bonide Mancozeb (with Zinc) and Immunox. Follow the label directions! Then begin a spray program, spraying first with Mancozeb, copper fungicide, or Daconil and then 10 days later spray with Immunox or one of the other fungicides that you didn't use for the first spray. Repeat this every 10 days until they flower.
Always read and follow the label directions when using any fungicide.